This invention relates generally to deep drilling tools for workpieces and more particularly, it relates to a deep drilling tool of the type having a drill bit supporting at least one cutting edge, and a drill tube or stem connectable to the drill bit. The drill stem communicates with an orifice of the drill bit and the free end of the drill stem is provided with an inlet for introducing a pressure fluid into the borehole.
In German Pat. DE-PS 767 138 granted to applicant, a tubular deep drilling stem of a symmetrical cross-section has been described that has found broad application all over the world and has almost fully replaced, in the range of drilling diameters over 10 mm, the conventional deep-drilling stems having a rolled-in longitudinal outer groove for discharging drilled chips. The drilling stem according to this patent feeds the drilling pressure fluid by means of a "BOZA" apparatus (drilling oil feeding apparatus) into the annular space between the wall of the bore hole and the outer surface of the deep drilling stem to the drill bit. In the area of the drill bit the fluid takes along drilling chips and flows on past a cutting edge into an abruptly increased interior space of the deep-drilling stem where the flow velocity of the fluid as well as of the chips carried therewith is abruptly decreased; at this low speed the mixture is discharged at the end of the stem out of the bore hole. The larger the cross-section of the area to be drilled in the bore hole by the drilling tool, the larger the annular space which has also to be provided in order to attain sufficient speed of the pressure fluid in the drill bit orifice and, consequently, to insure an instant withdrawal of the chips from the critical drilling area. As a rule, in the case of a rotating workpiece and stationary drilling tool as well as in a reversed or combined mode of operation, the annular cross-sectional area between the wall of the bore hole and the stem pipe has to be about 25% larger than the cross-sectional area of the drill bit orifice because even at a depth of the bore hole of about 20 times the diameter of the bore hole, a pressure drop due to the relative rotary motion between the walls of the annular space and due to the resulting heating of the pressure fluid caused by this motion, takes place. For this reason, a drilling stem having a relatively small diameter in comparison to the required diameter of the bore hole and having also relatively thin walls, has to be employed. Consequently, the resistance of the shaft against lateral flexture, against torsion and especially against torsional swing is relatively small. As a result, it is necessary to operate at a reduced drilling output and at a reduced drilling or cutting speed, especially when hard metal cutters having a long life are employed. Naturally, the economy of deep drilling is impaired.
A deep drilling tool is known from the German Pat. DE-PS 14 77 730 in which the drilling stem consists of two concentric tubes. The annular space between the two tubes serves for feeding the pressure fluid into the bore hole and the mixture of the pressure fluid with the drilling chips is discharged through the inner tube. It is true that in this embodiment it is possible to place the exterior pipe of the shaft relatively close to the wall of the bore hole; nonetheless the cross-sectional area of the annular space has to be maintained relatively large in order that the undesirable pressure drop of the pressure fluid caused by friction on the relatively large wall areas of the annular space, be prevented. In view of the fact that the cross-sectional area of the inner tube has to be maintained at a minimum value just sufficient for a reliable discharge of drilling chips, the desired radial extension of the annular space can be made only by limiting the thickness of the walls of the concentric tubes. The disadvantage of such measures, however, is the fact that the strength and mechanical resistance of the drilling stem is limited and such limitation, especially in deep bore holes, is undesirable.
In another prior art deep drilling tool according to my German Pat. DE-PS 1 290 026 the stem, similarly as in the previously discussed DE-PS 14 77 730, has two concentric pipes. The inner pipe, however, is connected to the outer pipe under torsional bias to form a torsional unit. Nonetheless, as far as the feeding of the pressure fluid is concerned, the same limitations as in the DE-PS 14 77 730 also obtain.